The gray dwarves (Duergar) are an evil and bitter race of dwarves who dwell deep in the Underdark. Male and female duergar are bald, and women do not grow beards. They are much thinner than other dwarves, with severe facial expressions, gray hair and gray skin. Duergar on the whole are evil, but some turn their backs on their fellows and seek a different sort of life. The surface dwarves hate the duergar because they turned to evil, and no other surface races hold much love for the gray dwarves. Most of the gray dwarves met by surface dwellers are tattooed exiles who were marked by the duergar community as criminals.

[Source: Forgotten Realms Campaign Setting by Greenwood, Reynolds, Williams, and Heinsoo, published by Wizards of the Coast, page 11-12.]